Wind turbine generators, which convert wind force to electric power, are operated in various locations, including locations adjacent to areas where acoustic noise is regulated. However, when a rotor head having blades is rotated, acoustic noise is generated in some cases, and, in these cases, the acoustic noise needs to be suppressed (reduced).
A conceivable method of suppressing the acoustic noise is to reduce the tip speed of the blades. To reduce the tip speed, the rotational speed of the blades needs to be reduced.
In order to suppress the acoustic noise, an acoustic-noise suppression operation for reducing the rotational speed of the blades is performed in some cases.
Depending on the characteristics of the blades of a wind turbine generator, acoustic noise generated through the rotation of the blades is reduced more when the pitch angles of the blades are set at a feathered side than at a fine side.
A graph in FIG. 7A showing the relationship between the wind speed and the rotational speed of the blades is obtained in an example case where the pitch angles are controlled so as to be set at the feathered side, thereby suppressing an increase in the rotational speed of the blades.
The line A (solid line) in FIG. 7A shows an example case of a normal operation in which the pitch angles of the blades are controlled such that the rotational speed of the blades is set constant at 15 rpm when the wind speed reaches 8.5 m/s. In this case, however, acoustic noise is generated. Therefore, in an acoustic-noise suppression operation shown by the line B (dashed line), when the wind speed is about 7.5 m/s or more, the pitch angles are set at a more feathered side than in the normal operation such that the level of the acoustic noise becomes 105 dB, for example. Accordingly, the rotational speed of the blades is reduced more than in the normal operation when the wind speed falls in a range from about 7.5 m/s to 14 m/s, and it reaches 15 rpm when the wind speed becomes 14 r/s.
PTL 1 discloses a technology of reducing acoustic noise while suppressing deterioration in the performance of a wind turbine generator by setting the pitch angles at a feathered side. PTL 1 describes a technology in which a blade pitch-angle control section calculates, for each wind-turbine blade, a first pitch-angle command value from the target pitch angle and the actual pitch angle of the wind-turbine blade, calculates, for each wind-turbine blade, an acoustic-noise index value by using a function having parameters of the rotational speed of the wind-turbine blades and the distance from a predetermined observation point set in advance to the wind-turbine blades, and corrects the first pitch-angle command value of a wind-turbine blade whose acoustic-noise index value is equal to or larger than a predetermined threshold, to a feathered-side value.
{Citation List}
{Patent Literature}
    {PTL 1}Japanese Unexamined Patent Application, Publication No. 2010-159646